1. Field of the Invention
The present invention relates to a data process to process data, such as a file inputted by a scanner or a PC, into general compressed image data and process the compressed image data to be outputted. More specifically, the invention relates to an image processing apparatus that performs an output image process to remove noise generated in restoring and outputting the compressed image data.
2. Description of the Related Art
In recent years, processing of document images has changed from analog processes to digital processes due to the development of reading by line sensors formed of CCD (Charge-Coupled Devices) units and laser printing. Digital multifunction peripherals are widely known as image processing apparatus using such techniques.
A digital multifunction peripheral performs various functions such as a scanner function, a printer function, a facsimile (FAX) function, and the like in addition to a copying function. A processing unit commonly used by these functions is provided in the digital multifunction peripheral. A machine incorporating these functions is predominantly used and called a digital multifunction peripheral (MFP: Multifunction Peripherals), which controls the multifunction (see Patent Document 1).
The multifunction (functions of a printer, a scanner, internet FAX, and the like) of the digital multifunction peripheral connected to the network can be used by external devices on the network.
Therefore, outputs from the MFP include an output onto paper for copying and an output by sending electronic data for scanner and FAX delivery. Also, in sending electronic data, there are different output formats depending on the intended purpose. For example, a binary image data format is used for the FAX delivery, while a color RGB image data format, for example, is used for the scanner delivery. Moreover, the MFP outputs image data by different output units such as a writing unit in the paper output and a display of a PC (Personal Computer) to display the delivered image data in the scanner delivery. Therefore, appropriate processes are required for each output.
The output image data and the like processed when the multifunction such as a copying function is used through the network are stored in an internal HDD (Hard Disc) or the like so that the stored image data can be reused and outputted.
Even the HDD with high capacity, however, has a limit in its data storage capacity. When printing a full color image with a resolution of 600 dpi, for example, a color image of one A4 page has quite a large data volume of about 120 MB.
Thus, in the MFP having an output unit capable of printing color images, color image data are compressed when stored in a storage device such as an HDD and expanded in the MFP when printing the expanded image data so that time required to transfer data becomes shorter and the required memory capacity becomes smaller; thereby making processes more efficient and the structure simpler. This data compression is employed not only in the MFP but for data sent outside the MFP through the network for similar effects.
A compression method of image data may be a lossless compression method for a multifunction peripheral which processes mainly documents of monochrome text or line drawings. To process color image data, however, a more general lossy compression method such as JPEG (Joint Photographic Expert Group) is better in many cases.
The JPEG compression method is effective for inputted images such as photographs, however, there is a problem that mosquito noise is generated in a line drawing such as a CG (Computer Graphic) image, text, and CAD (Computer Aided Design) restored by expansion. As a result, image quality of the restored image is degraded.
Patent Documents 2 and 3 disclose the related arts to solve the aforementioned problems.
Patent Document 2 discloses a method to remove noise generated in expanding a data-compressed image by applying an appropriate filter process determined by a compression rate and an image attribute such as text, a graphic image, and halftone dots to image data after decompression.
Patent Document 3 discloses a method to find whether a target pixel and its peripheral pixels of an expanded image match a predetermined noise detection bit map pattern, and if they match, apply a correction to a noise pixel to remove the noise.    Patent Document 1: Japanese Patent No. 3647347    Patent Document 2: Japanese Patent Application Publication No. 2001-211318    Patent Document 3: Japanese Patent Application Publication No. 2002-354257
In Patent Document 1, however, noise is removed by the filter process, which is not very effective in reducing noise depending on the image data. In Patent Document 2 also, noise is removed by using the predetermined noise detection bit map pattern, therefore, a precision of noise detection is sometimes low depending on the detected image data, which means this method is not very effective in reducing noise depending on the image data.